1. Field of the Invention
The invention relates generally to a system and method for improving telecommunications and, more particularly, to a system and method for reliably processing changes in signaling bits in multichannel telecommunication lines sent over a global network.
2. Description of the Prior Art
Digital carrier technology is the predominant means of transmitting voice communication over global computer networks in use today. The North American T-carrier system of voice transmission, introduced in the 1960s by Bell System, operates using the Digital Signal (DS) series as a base multiple. DS is a term used for the series of standard digital transmission rates or levels based on DS0, a transmission rate of 64 Kbps. 64 Kbps is the standardized bandwidth typically associated with a single telephone voice channel. DS1, used as the signal in the T1 carrier, comprises 24 DS0 (64 Kbps) signals, channels, or timeslots transmitted using pulse-code modulation (PCM) and time-division multiplexing (TDM). The DS1 frame has a duration of 125 μsec.
In the T1 system, voice signals are sampled 8,000 times a second and each sample is digitized into an 8-bit word. With 24 timeslots being digitized at the same time, a 192-bit frame (24 timeslots each with an 8-bit word) is thus being transmitted 8,000 times a second. Each frame is separated from the next by a single framing bit, resulting in a 193-bit block. The 192-bit frame multiplied by 8,000 and the additional 8,000 framing bits make up the T-1's 1.544 Mbps data rate. The signaling bits are the least significant bits per frame.
The E1 system is the European format for digital transmission. In the E1 system, signals are carried at 2.048 Mbps (32 timeslots at 64 Kbps). E1 and T1 lines are often interconnected for international use.
Channel Associated Signaling (CAS) refers to signaling in which control signals, such as those for synchronizing and bounding frames, are carried in the same channel or timeslot along with the voice and data signals. In DS1 CAS, control information is embedded in signaling frames where the least significant bit of each DS0 within the frame is robbed for signaling. Signaling frames are found every 6th frame. For AB signaling, frame 6 carries the A bit and frame 12 carries the B bit. For ABCD signaling, frames 6, 12, 18, and 24 carry the corresponding ABCD bits. The 24 frames form what is known as Extended Super Frame or ESF. CAS signaling is used for call set up and termination. Signaling is accomplished by detecting changes in the ABCD bits.
In E1, the signaling bits are presented in timeslot 16 of frames within a E1 multiframe.
FIG. 1 is a block diagram of a conventional signaling system 10 for processing changes in signaling bits for telecommunication transmitted over a global network. The signaling system includes framers 1, 2, . . . , N for receiving corresponding DS1/E1 signals.
The framers 1, 2, . . . , N extract the signaling bits from the DS1/E1 signals, store them in corresponding buffers 14A, 14B, . . . , 14N, and present them to processor 12 for further processing. Buffers 14A, 14B, . . . , 14N are often sized to allow one entry for each channel, timeslot, or DS0. Thus, buffers 14A, 14B, . . . , 14N shown in FIG. 1 have, e.g., 24 registers, one for each of the 24 timeslots making up a single DS1 signal. As new events arrive with signaling bit changes for a particular timeslot on a corresponding DS1/E1 signal, the framers 1, 2, . . . , N overwrite the pre-existing entry. Alternatively, the framers 1, 2, . . . , N each include a fixed buffer (not shown in FIG. 1) that is overwritten every time a new bit changes in an ESF.
In both cases, signaling bits can be lost before being processed by the processor 12. In the first case, if a new event arrives for a timeslot before the microprocessor 12 services the preexisting event, the preexisting event is lost. In the second case, the microprocessor 12 must process all events before a new DS1/E1 signal arrives with its consequent bit changes, e.g., within 3 ms.
Lost signaling bits result in unsuccessful call set up and termination. Lost signaling bits increase the incidence of dropped calls. Lost signaling bits also cause problems to higher layers of the signaling stack as changes are simply overwritten without notification. Finally, lost signaling bits increase call retries adversely affecting the operating speed.
Accordingly, a need remains for a scalable system and method for reliably sequencing signaling bit changes in telecommunications transmitted over a global network.